Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Various oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol, are alternative feedstocks for the production of light olefins. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
In an oxygenate to olefin (OTO) reaction system, an oxygenate in an oxygenate-containing feedstock contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenate to light olefins, which are yielded from the reaction system in a reaction effluent. As the feedstock contacts the molecular sieve catalyst compositions at high weight hourly space velocities and under elevated temperature and pressure conditions, a portion of the catalyst compositions can break up, e.g., attrit, to form one or more smaller attrited catalyst particles. Some catalyst attrition particles are very small in size and are referred to as catalyst fines. Due to their relatively high surface area to mass ratios, a portion of the catalyst fines in the reaction system may become entrained with the reaction effluent and exit the reaction system therewith. Conversely, due to their relatively low surface area to mass ratios, larger particles tend to be selectively retained in OTO reaction systems. The selective retention of larger particles is particularly a problem for highly attrition resistant particles.
Additional catalyst particles, particularly catalyst fines, may be lost from a catalyst regenerator associated with the OTO reaction system. In conventional regeneration vessels, coked catalyst is directed from a reactor to a catalyst regenerator. In a catalyst regenerator, the catalyst is fluidized with a fluidizing medium, typically comprising the regeneration medium, under conditions effective to remove the coke from the coked catalyst by combustion with the regeneration medium and form regenerated catalyst and gaseous byproducts. The bulk of the regenerated catalyst from the regenerator is returned to the reactor. The gaseous byproducts are forced out an exhaust outlet oriented in the upper section of the catalyst regenerator. Undesirably, a significant amount of entrained catalyst from the regenerator, particularly catalyst fines, is forced out the exhaust outlet with the gaseous byproducts.
One undesirable effect caused by the build up of large catalyst particles in an OTO reaction or regeneration system is that in a large particle rich reaction or regeneration system, the circulating fluid bed or fluidized bed will not operate as well due to the loss of the fine particles, particularly with regard to circulation of catalyst within the standpipes and also with regard to gas distribution within the fluidized bed. Several processes have been disclosed that address this problem.
One conventional technique, for example, includes non-selectively removing a fraction of all of the catalyst particles in the reaction system to make room for the addition of fresh catalyst. This technique for removing undesirably-sized catalyst particles is inefficient, however, because a significant portion of the desirably-sized catalyst particles are removed from the reaction system with the undesirably-sized catalyst particles.
U.S. Pat. No. 5,746,321 to Hettinger, Jr. et al. discloses the combination of a magnetic separator, a catalyst classifier, and/or a catalyst attriter, which wears off the outer layers of catalyst, yields more active catalyst of lower metal content with closer control of average particles size, and narrows particle size distribution providing improved fluidization properties and better activity and selectivity. The process is especially useful when processing high metal-containing feedstocks.
U.S. Pat. No. 2,573,559 to Friedman discloses replacing a bed of fluidized catalyst, which has become reduced in activity during use, with fresh fluidized catalyst, the average particle size of both catalysts being in the range of 40-400 mesh. The average size of the fresh catalyst differs from that of the partially spent catalyst by at least 10-mesh size and preferably 25 mesh. The fresh catalyst is introduced into the reactor under conditions such that the reaction temperature is not substantially increased, and at the same time catalyst is withdrawn from the reactor at a portion below the top level of the bed. The catalyst withdrawn is separated by particle size into fresh catalyst, which is returned to the reactor, and deactivated catalyst, which is regenerated. According to the '559 patent, the complete replacement of catalyst can be accomplished under normal operating conditions in from 20 to 48 hours.
U.S. patent application Ser. No. 10/835,494, filed on Apr. 29, 2004, discloses processes for maintaining a desired particle size distribution in an OTO reaction system. In one embodiment, the invention comprises replacing lost catalyst fines with less active co-catalyst particles. By adding less active co-catalyst particles to the reaction system, desirable fluidization characteristics and hydrodynamics can be maintained without affecting the overall performance and product selectivities.
The need remains, however, for improving the fluidization characteristics of a population of catalyst particles that is depleted of catalyst fines, e.g., has a high particle size distribution or median particle diameter, without necessitating the addition of new catalyst fines.